Cyclic polymers have been prepared by any number of synthetic routes, but standard techniques typically focus on the formation of end-functionalized linear polymers that must be cyclized under high-dilution conditions. Tethered-alkylidene variants of standard metathesis catalysts open the door to high molecular weight cyclic polymers and high-concentration polymerization reactions through ring expansion metathesis polymerization (REMP).
REMP catalysts feature short hydrocarbon tethers that create metallocycles with the alkylidene. Ring-opening of each cyclic monomer creates a larger and larger metallocycle. Chain transfer to the alkene closest to the tether point regenerates the initial catalyst, releasing the cyclic polymer. Alternately, chain transfer to any other alkene in the metallocycle “pinches” off a portion of the ring as a free cyclic polymer, leaving a smaller catalytic metallocycle that is free to grow again.
Since publishing initial studies in 20021, the Grubbs group has demonstrated how minor changes in catalyst structure can have dramatic effects on the kinetics of polymer growth. Recent work2, 3 shows how catalysts with six-carbon tethers propagate faster than they release or pinch (giving rapid molecular weight growth), while catalysts with five-carbon tethers have competitive rates of propagation and catalyst release (giving step-like growth of the molecular weight). Once all monomer is consumed, continuing inter- and intramolecular chain transfers cause the polymer molecular weights to equilibrate, based on the most stable ring sizes for the reaction conditions.
The Grubbs group has already used this technique to access several cyclic polyolefin and polyalkane structures that have been difficult to achieve by other means, including novel dendronized structures4. What will be the next novel polymer made possible by REMP?
1 Bielawski CW, Benitez D, Grubbs RH. “An ‘Endless’ Route to Cyclic Polymers,” Science, 297, 2002, p2041.
2 Boydston AJ, Xia Y, Kornfield JA, Gorodetskaya IA, Grubbs RH. “Cyclic Ruthenium-Alkylidene Catalysts for Ring-Expansion Metathesis Polymerization,” JACS, 130, 2008, p12775.
3 Xia Y, Boydston AJ, Yao Y, Kornfield JA, Gorodetskaya IA, Spiess HW, Grubbs RH. “Ring-Expansion Metathesis Polymerization: Catalyst-Dependant Polymerization Profiles,” JACS, 131, 2009, p2670.
4 Boydston AJ, Holcombe TW, Unruh DA, Fréchet JM, Grubbs RH. “A Direct Route to Cyclic Organic Nanostructures via Ring-Expansion Metathesis Polymerization of a Dendronized Macromonomer,” JACS, 131, 2009, p5388.